The present invention relates to a method of separating catalyst-free working solution from noble metal black catalyst-containing working solution, especially from palladium black catalyst-containing working solution in the hydrogenation stage of the so-called anthraquinone process for the production of hydrogen peroxide. The separation is carried out by filtration of the catalyst-containing working solution using filter candles.
As is known, in the so-called anthraquinone process for the production of hydrogen peroxide, also called the AO process, a reaction carrier based on one or several 2-alkylanthraquinones and tetrahydro-2-alkylanthraquinones in an organic solvent system is converted with hydrogen in the presence of a catalyst into the corresponding hydroquinone form. After the hydrogenation stage the working solution freed from catalyst is treated in the oxidation stage with an oxygen-containing gas, at which time the quinone form of the reaction carrier re-forms under formation of hydrogen peroxide. Finally, hydrogen peroxide is extracted with water from the oxidized working solution and the working solution, that is, the mixture of the reaction carrier and solvent or solvent mixture, is returned to the hydrogenation stage. Essential details of the AO process can be gathered from Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. (1989), vol. A 13, pp. 447-456. See also Kirk Othmer, Encycl. of Chem. Tech. 3rd Ed., Vol. 13, pages 16 to 21 relied on and incorporated herein by reference.
Frequently, catalysts based on noble metals are used as hydrogenating catalysts, which can be carrier-bound or carrier-free suspension catalysts or fixed-bed catalysts. The cycle of the hydrogenation stage employing suspension catalysts includes essentially the actual reactor, a circulating flow line with circulating pump, a device for the supplying and distributing of the hydrogen, a device for the supplying of working solution from the working-solution drying stage which is connected in after the extraction stage. Also included is a device for the removal of catalyst-free, hydrogenated working solution, which last-named device is a solid-liquid separating device.
The quantitative retention of the suspension catalyst in the hydrogenating cycle, which is free of problems in continuous operation, and the separation of catalyst-free working solution from the latter are basic prerequisites for a reliable and economical process. The required solid-liquid separation is based on a filtration using fine-pored filters which are periodically back-flushed.
According to the method of Canadian patent 1 208 619, in the anthraquinone method for the production of hydrogen peroxide, suspension-catalysts with a particle size range of 75% greater than 1 .mu.m such as e.g. Raney Nickel can be retained using filter elements of sintered metal particles with a maximum pore width of 8 .mu.m. This method is not suited for separating a catalyst-free, hydrogenated working solution from a working solution containing palladium black because Pd black normally exhibits a primary particle size range between 5 and 50 nm and such a sintered-metal filter is on the one hand not tight against palladium black and on the other hand rapidly blocks up--see comparative example 3 in DE-OS 42 17 245.
According to U.S. Pat. No. 3,433,358 filter candles of carbon material, so-called carbocandles, are suitable for separating a catalyst-free, hydrogenated working solution from working solution containing Pd black, the pore width of which candles can be greater than the diameter of the Pd particles to be separated. The wall thickness of such filter candles acting on the principle of deep-bed filtration is indicated to be at least 10 mm. U.S. Pat. No. 3,433,358 does speak of a surface filtration; however, tests by applicants herein showed that a filter thickness like that in customary membrane filters would be totally insufficient in the case of carbocandles. On account of the limited mechanical stability of such carbocandles narrow limits are imposed on the design; the required filtration performance results in rather large filter housings and therewith in a high capital investment. There is an interest in more effective and more stable filter elements, especially in those with improved mechanical stability and increased filtration performance in order to increase the economy of the AO method.
Previously mentioned U.S. Pat. No. 3,433,358 also teaches that carbocandles are far superior to metallic and ceramic filter materials. This statement is also supported by DE-OS 41 29 865, which is about 25 years younger. According to it when filter elements based on a metallic or ceramic sintered product are used in generic methods utilizing palladium black (=Pd black) a special, very expensive purification method is necessary in order to render the filter element reusable and to obtain a technically acceptable filtration performance. The purification method reduces the economy of the AO method.
As is known from DE-OS 42 17 245, Pd black can also be retained in the AO method using a microfilter operated according to the cross-flow principle with a filter membrane of ceramic material with a pore width of advantageously 0.1 to 1.0 .mu.m and catalyst-free, hydrogenated working solution can also be separated from the hydrogenation cycle.
A disadvantage in the previously cited method with cross-flow filtration technology is the fact that in practice the average pore width of the membrane is rather low, preferably between 0.1 and 0.5 .mu.m, resulting in a limited filtration performance. In addition, it turned out when such filter elements were continuously operated that the filtration performance drops slowly but continuously, which can not be avoided by increased back-flushing.
Ceramic filter elements are presented under characteristic ID number 314 in Verfahrenstechnik 26 (1992), No. 6, 30 which exhibit asymmetric particulate ceramics. The area of use is indicated as catalyst separation and recovery, among other things in the production of hydrogen peroxide. No suggestions result either from this document nor from the manufacturer about whether the catalysts are the carrier bound noble-metal ones predominantly used in the AO method or are the non-carrier-bound noble-metal catalysts, which are considerably more difficult to filter. The filter elements presented exhibit a support body with an average pore width of 40 .mu.m and a membrane with an average pore width of 1.5 or 3 or 5 .mu.m. Taking into consideration the previously indicated state of the art for catalyst separation in the AO method an expert in the art would relate the "Verfahrenstechnik 26 (1992), No. 6" document only to those AO methods in which the pore width of the active filter element is approximately the same as or smaller than the particle size of the catalyst particles to be separated. An expert in the art would hardly consider the cited filter candles for AO methods using Pd black as a suitable catalyst because the pore width of the membrane (1.5 to 5 .mu.m) is a multiple larger than the diameter of the primary particles of Pd black (5 to 50 nm) and, in addition, insufficient tightness against Pd black, a danger of blocking up and a drop in the filtration performance must be reckoned with.